allow for GA options passed through

[blairanderson]

GA field reference: https://developers.google.com/analytics/devguides/collection/analyticsjs/field-reference

react-ga does a good job with this: https://github.com/react-ga/react-ga/blob/master/src/core.js#L76-L80

(Its an extra object that gets passed through on each page view. for sessions and )